|dc.description.abstract||Glass and Carbon Fibre Reinforced Polymer (GFRP/CFRP) composites are currently being utilised widely in an increasing range of engineering applications due to their chemical re- sistance, design flexibility, and high stiffness-to-weight and high strength-to-weight ratios during the last few decades. However, the failure of composites is difficult to predict as the onset of damage in the composites does not result in the catastrophic failure of the whole structure instantaneously but progressively because the collective and interactive damages can transform from one mode to another across the length scales. The World-Wide Failure Exercises (WWFE-I, II, III) assessed the most widely used failure criteria and concluded that most of them could only predict the ultimate strength of composites accurately under some loading conditions, however, none is capable of predicting progressive failure process in the composites. Classical continuum mechanics based Finite Element Method (FEM) has been used to tackle the damage propagation problem by setting a degradation factor for several decades, based on the assumption that the material does not fail. Besides, the newly proposed Extended FEM (XFEM) needs a predefined crack path for the crack propagation, lacking the randomness characteristic of real failures. Therefore, a 3D meso-scale Discrete Element Method (DEM) model is developed for unidirectional FRP composite materials to predict the elasticity and strength of FRP composites. With the help of cross-validation between two numerical approaches and experimental findings under static loading conditions, the failure progression problem can be better addressed.
To achieve this goal, firstly, a 3D FEM based micro-scale Representative Volume Element (RVE) model which represents the microstructure of composite laminae is developed. This model is utilised to model the mechanical behaviour of FRP composite materials subjected to different combined loading conditions by using Periodic Boundary Conditions (PBC). The Drucker-Prager plastic constitutive material model and the Ductile failure initiation and evolution criteria are applied to simulate the plastic and damage process of matrix. A bilinear mixed-mode softening law is utilised to simulate the mechanical response of the interface between fibre and matrix. In addition, here in this study, fibres are assumed to be transversely isotropic elastic. A weak and a strong interface are considered, and the numerical results are compared to the theoretical results predicted by three popular failure criteria, such as Hashin, Tsai-Wu and Puck failure criteria. An assessment has been made between these criteria, and the Tsai-Wu failure criterion stands out due to its more general formulation and applicability in different cases.
Secondly, three different 3D meso-scale DEM based models are developed for the prediction of elasticity of transversely-isotropic materials considering different packing patterns, namely the 3D lattice discrete model, the 3D Hexagonal Close Packing (HCP) model and the extended 2D hexagonal and square models. These DEM models have been validated and assessed by theoretical analysis, FEM simulations and experimental results available in the literature. The extended 2D hexagonal and square models are based on average strain energy method, and are selected for the prediction of progressive failure of the FRP composite laminae in the next stage due to their simplicity, accuracy and relatively short computation time.
Thirdly, the bond strengths of the extended 2D DEM hexagonal model of 0◦ and 90◦ com- posite laminae are calibrated from experiments. In contrast, the bond strength of the extended 2D DEM cubic model of 45◦ lamina is calibrated from the failure prediction via the Tsai-Hill failure criterion under plane stress state. The total strain energy release rate is considered for the interfacial bond to model the delamination of composite materials. Quantitative analysis of progressive damage is conducted for a cross-ply composite laminate, including crack density and stiffness degradation with the validation of experimental findings in the literature. Qualita- tive analysis of an Open-Hole Tension (OHT) case is conducted on a quasi-isotropic composite laminate regarding its damage initiation and propagation process with a comparison to the experimental findings, such as Micro-CT images.
Finally, a seven-bond interface model is developed based on the energy balance principle and a power-law relation of bond lengths in the interface for the simulation of progressive delamination process in DCB tests. It has been validated that this model is capable of predicting the stiffness and ultimate peak load accurately comparing with experimental findings. Further, this model is adopted for the construction of CFRP cross-ply composite laminates. Experiments are conducted for the validation of the improved DEM model regarding the failure prediction of the CFRP composite laminates, and relatively good agreements are found between the results of the experiments and numerical simulations.||en